The invention relates to continuously monitoring the concentration of potentially explosive materials, such as hydrocarbon, in a number of separate gas streams used as samples discharged from gas containing equipment such as coating installations. More particularly, the invention relates to an arrangement for continuously monitoring the concentration of potentially explosive gases in gas streams by means of flame-ionizers where the concentration at a lower limit of explosiveness is monitored by ionization of gas samples.
The application of flame-ionization detectors for monitoring the concentration of hydrocarbon in gases or air is known in the art.
In such a detector or flame-ionizer, a fuel gas and a combustion supporting gas, such as air or oxygen are fed into a flame-ionizer chamber where they are burned to produce a flame. A gas stream of a substance to be tested containing a certain amount of hydrocarbon is supplied into the chamber, which gas stream passes through the flame of the ionizer which is provided with two electrodes supplied with direct voltage potential.
An electric current passing between the electrodes of the ionizer while a sample gas stream is passing through the flame is a function of the concentration of hydrocarbon in the sample gas stream. This current is approximately directly proportional to the amount of hydrocarbon atoms passing through the flame per time unit.
The flame-ionizers may be utilized, for example for monitoring the concentration of exhaust gases in motor vehicles, or also in the chemical or petro-chemical industry.
The flame-ionizers may be also employed in fields of technology where potentially explosive gases or vapors may be contained in the working gases and where the concentration of such potentially explosive gases should be monitored. One of such fields is coating installations.
Examples of such installations may occur in the electrical industry where wires are coated with insulating material, in the furniture industry where chipboard panels are covered with veneer, or in the construction industry where construction panels or frames are covered with synthetic plastic material; and also in the packing industry where sheets of carrier material are laminated at one side or at both sides thereof with a layer of synthetic plastic material.
In all of these applications, a coated material is passed through drying kilns or ovens in which solvents utilized in the coating materials are expelled in the form of vapors from the materials being dried during the drying process and must be removed. These solvent vapors may be highly explosive and are normally forwarded to after-burning equipment where they are burned, or else they may be directed to a solvent recovering installation.
In these applications care must be taken that the solvent vapors do not ignite, since this may lead to heavy explosions and to the destruction of the entire coating installation. To prevent this, the concentration of potentially explosive solvents in the vapors must be kept sufficiently low so that it cannot reach the so-called lower limit of explosion.
The drying installations known in the art normally include a number of individual drying chambers in which solvent vapors of different degrees of concentration evolve; all of these concentrations must be monitored to avoid the aforementioned undesirable results.
It has been proposed to monitor the gases or vapors evolving in the individual drying chambers by monitoring their heat content. The disadvantage of this method is that catalysts utilized in this method react relatively slowly and that the response time of such catalysts to changes in the concentration of the potentially explosive substances in the gases takes several seconds. Furthermore, this method has been found not to be sufficiently sensitive when only relatively small proportions of hydrocarbon are present in the gases to be monitored. Additionally, the catalysts are poisoned by certain substances contained in lacquer raw materials, such as heavy metals or sulfur-compound materials whereby the service life of the catalysts is reduced to only a few hours. Also, in the event silicones are present the catalysts or carrier material may become clogged by the silicone during the monitoring process.
It has been further suggested to withdraw a sample gas stream from each drying chamber and to pass these streams through a common pipe, by means of corresponding control valves, one after another to and through a flame-ionizer in which the concentration of hydrocarbons in the individual sample can be monitored.
This method has also been found disadvantageous in that it is evidently impossible to avoid discontinuous monitoring of the hydrocarbon concentration. Furthermore, only a portion of the time allotted for the monitoring of each separate sample is actually available for the monitoring operation, since the residual amounts of gas from the previous sample must be flushed out of the conduits and the flame ionizer before the next sample is admitted to prevent the occurrence of measuring errors due to commingling of two samples. When, for example, a new sample is admitted every ten seconds, then eight seconds are required to flush the previous sample out of the conduits and the ionizer and only two of the ten seconds are available for the actual measuring operation per sample. Even so, it is not possible in this method to exclude with absolute certainty that parts of the preceding sample may remain in the conduits and mingle with the next-following sample. Finally, the systems for switching the different sample-furnishing chambers into and out the sampling circuit requires relatively complicated and expensive electrical and/or electronic equipment, including data storages to provide quasi-continuous control.